In such a process for preparing synthesis gas an oxygen-containing gas, which is applied as an oxidiser, and a hydrocarbon-containing fuel are supplied to a gasification zone through a burner comprising an arrangement of passages or channels for fuel and oxidiser, and wherein autothermically a gaseous stream containing synthesis gas is produced under appropriate conditions. The passages or channels are separated from each other by separation walls, the so-called burner internals.
In many known processes for preparing synthesis gas a moderator gas (for example steam, water or carbon dioxide or a combination thereof) is supplied to the gasification zone in order to control the temperature in the gasification zone. Moderator gas may be added to the oxidiser or (gaseous) fuel or may be supplied via a moderator gas passage. Those skilled in the art will know the conditions of applying oxidiser and moderator gas.
Advantageously, a multi-orifice (co-annular) burner comprising a concentric arrangement of n passages or channels co-axial with the longitudinal axis of said burner, wherein n is an integer &gt;2, is applied.
Such multi-orifice (co-annular) burners contain substantially cylindrical internals which separate the fluid streams flowing through the passages until they reach the burner exit. In particular, such multi-orifice (co-annular) burners comprise an arrangement of annular concentric channels or passages for supplying oxidiser, moderator gas (optionally) and fuel to the gasification zone.
Multi-orifice (co-annular) burners are known as such and the mechanical structures thereof will therefore not be described in detail.
Usually such burners comprise a number of slits at the burner outlet and hollow wall members with internal cooling fluid (e.g. water) passages. The passages may or may not be converging at the burner outlet. Instead of comprising internal cooling fluid passages, the burner may be provided with a suitable ceramic or refractory lining applied onto or suspended by a means closely adjacent to the outer surface of the burner (front) wall for resisting the heat load during operation or heat-up/shut down situations of the burner. Advantageously, the exit(s) of one or more passages may be retracted or protruded with respect to the outer passage.
It will be appreciated by those skilled in the art that any slit width suitable for the purpose can be applied, dependent on the burner capacity.
Advantageously, the central passage has a diameter up to 70 mm, whereas the remaining concentric passages have slit widths in the range of 1-30 mm.
However, it will be appreciated that the present application is not confined to the use of co-annular burners.
The oxidiser and the fuel and, optionally, moderator gas are supplied to the gasification zone through the respective channels at specific velocities and mass distribution in order to obtain a good atomization and mixing.
Advantageously the respective velocities are measured or calculated at the outlet of the said respective channels into the gasification zone. The velocity measurement or calculation can be carried out by those skilled in the art in any way suitable for the purpose and will therefore not be described in detail.
The oxygen-containing gas, which is applied as oxidiser, is usually air or oxygen or a mixture thereof. An oxidiser comprising at least 95% oxygen is preferred.
Synthesis gas is a gas comprising carbon monoxide and hydrogen, and it is used, for example, as a clean medium-calorific value fuel gas or as a feedstock for the synthesis of methanol, ammonia or hydrocarbons, which latter synthesis yields gaseous hydrocarbons and liquid hydrocarbons such as gasoline, middle distillates, lub oils and waxes.
In the specification and in the claims the term gaseous (liquid) hydrocarbon-containing fuel will be used to refer to hydrocarbon-containing fuel that is gaseous (liquid) at gasifier feed pressure and temperature.
According to an established process, synthesis gas is produced by partially oxidising in a reactor vessel a gaseous fuel such as a gaseous hydrocarbon, in particular petroleum gas or natural gas, at a temperature in the range of from 1000.degree. C. to 1800.degree. C. and at a pressure in the range of from 0.1 MPa to 12 MPa abs. with the use of an oxidiser.
Synthesis gas will often be produced near or at a crude oil refinery because the produced synthesis gas can directly be applied as a feedstock for the production of middle distillates, ammonia, hydrogen, methanol or as a fuel gas, for example, for heating the furnaces of the refinery or more efficiently for the firing of gas turbines to produce electricity and heat.
For economic reasons it is often desirable to operate the burner without the application of a moderator gas. Further, in order to obtain a good mixing of fuel and oxidiser in the gasifier it is preferred to operate the burner under such conditions that an oxygen blast exists (i.e. the velocity of the oxidiser is substantially larger than the velocity of the fuel at the outlet of the burner). Those skilled in the art will know these conditions.
It will be appreciated by those skilled in the art that oxygen blast permits lower fuel supply pressures and enables reduction of compression cost.
Usually, the rim of a burner internal (i.e. that part of the burner internal which, when the burner is mounted in a reactor vessel, is directed to the gasification zone and terminates in a tip) is made of steel or low alloy steel.
However, when applying oxygen blast in the absence of moderator gas it has appeared that the rim(s) of the burner internal(s) separating fuel and oxidiser is (are) severely attacked by metal dusting phenomena so that serious burner damage will occur and the burner lifetime is restricted.
Metal dusting is a catastrophic carburization which occurs in industrial plants under conditions of high activity and low oxygen pressure in the temperature range 600-800.degree. C., leading to decomposition of steels into a mixture of powdery carbon, metal particles and sometimes carbides and oxide. Pitting or general metal wastage is observed, if the corrosion product was carried away by erosion through the gas flow.
It is an object of the invention to provide an economically feasible process for preparing synthesis gas which can be carried out over a long period without the need for many shut downs.